This study proposes to examine the effects of repeated or chronic stress in squirrel monkeys (Saimiri sciureus) and titi monkeys (Callicebus moloch). Chronic activation of the stress response can be induced by repeated exposure to seemingly innocuous events that pose no physical risk to the animals but to which the likelihood of habituation is small. Although many aspects of the response to acute stress has been characterized in these species, the ability to compensate for the glucocorticoid rise associated with the stress response has not been studied nor has the consequences of chronic exposure to stressors. I will separate animals from their social group and place them in a novel environment daily for eight weeks in order to examine both the transient and permanent effects of repeated psychological stress in adult monkeys. The proposed research aims first to describe the circadian rhythm of HPA activity in both species and to describe changes due to exposure to a single stressor. Next, I will examine the consequences of chronic exposure to repeated stress, which are hypothesized to include: 1) increased mean daily output of HPA hormones (including increases in free cortisol and ACTH and a reduction in CBG and bound cortisol), 2) reproductive suppression, 3) immune suppression, 4) decline in health, 5) hippocampal cell death and 6) disruption of social relationships. Finally, the research proposes to compare the effects of repeated stress in squirrel and titi monkeys: titi monkeys are predicted to show greater sensitivity to chronic stress than squirrel monkeys, especially with respect to transient changes in reproductive, immunological, and social processes. However, squirrel monkeys may be expected to exhibit more permanent changes owing to high levels of HPA hormones and a more prolonged response to each exposure to stressor. Testing both species will illuminate the respective roles of physiological predisposition and social relationships in mediating the consequences of chronic stress. To the extent that species respond similarly, I will be able to generalize the effects of chronic stress in social animals.